Process for preparing pentenals

ABSTRACT

In the process for hydroformylation of 1,3-butadiene with carbon monoxide and hydrogen to form pentenals, the improvement comprising effecting the hydroformylation with 1,3-butadiene dissolved in an inert reaction medium at 100° to 180° C. at 5 to 35 MPa in the presence of an aqueous solution of a rhodium/phosphine complex and optionally a further phosphine as catalyst.

STATE OF THE ART

As a by-product from crude oil refining, 1,3-butadiene is formed, forexample, in a naphthapyrolysis as constituent of the C₄ fraction, a rawmaterial available in large amounts for chemical syntheses. One way ofutilizing it is the reaction with carbon monoxide and hydrogen(hydroformylation) to form carbonyl compounds and further reaction toform hydroxymethyl compounds. The hydroformylation of 1,3-butadiene hasbeen widely studied. Owing to the presence of two conjugated doublebonds, the reaction products to be expected are not only dialdehydes andmonoaldehydes in their various structural isomeric forms, but alsounsaturated C₅ -aldehydes.

According to Fell et al, dialdehydes are formed in the hydroformylationof 1,3-butadiene in the presence of phosphine-modified rhodium catalysts(cf. Chem.-Ztg., Vol 99 (1975), 485 ff). The saturated C₅ -monoaldehydesare obtained as reaction products in the hydroformylation in thepresence of cobalt carbonyl catalysts, rhodium carbonyl catalysts orphosphine-modified rhodium carbonyl catalysts (cf. Fell et al,Forschungsbericht Bundesministerium Forschung und Technologie,BMFT-FB-T-84 064, Karlsruhe 1984).

In contrast to the synthesis of C₅ -aldehydes and C₆ -dialdehydes byreaction of butadiene with carbon monoxide and hydrogen, thehydroformylation of the 1,3-diolefin to give unsaturated C₅ -aldehydesproceeds with only low selectivity. In the reaction of the reactants ina homogeneous reaction system with rhodium carbonyl/phosphine catalysts,pentanals are preferentially formed even under changing reactionconditions. Although the hydroformylation of 1,3-butadiene in thepresence of water-soluble rhodium carbonyl/phosphine catalysts givesprimarily n-penten-3-al as reaction product, the highly reactiveunsaturated aldehyde is converted by aldol condensation into2-propenylheptadienal and further into C₁₅ - and higher molecular weightaldol products.

OBJECTS OF THE INVENTION

In view of the fact that unsaturated C₅ -aldehydes are of greatindustrial interest as intermediates capable of undergoing numerousreactions because of the presence of the aldehyde function and a doublebond in the molecule, it is an object of the invention to provide aprocess which allows the preparation of pentenals with high selectivityand yield.

This and other objects and advantages of the invention will becomeobvious from the following detailed description.

THE INVENTION

In the process for hydroformylation of 1,3-butadiene with carbonmonoxide and hydrogen to form pentenals, the improvement compriseseffecting the hydroformylation with 1,3-butadiene dissolved in an inertreaction medium at 100°to 180° C. at 5 to 35 MPa in the presence of anaqueous solution of a rhodium/phosphine complex and optionally a furtherphosphine as catalyst.

It has surprisingly been found that the selectivity of the conversion of1,3-butadiene by hydroformylation in a heterogeneous reaction system isconsiderably improved if the 1,3-butadiene is not used as a puresubstance, but is dissolved in a medium which is inert under thereaction conditions. The dilution of the diolefin leads, in particular,to avoidance of the subsequent reactions, viz. formation of aldolcondensation products. Suitable inert media are, in particular,saturated aliphatic and cycloaliphatic hydrocarbons of 3 to 10 carbonatoms in the molecule and also aromatic hydrocarbons of 6 to 9 carbonatoms in the molecule. Examples of compounds of the classes mentionedare propane, n-butane, n-hexane, cyclohexane, toluene and the xylenes.The use of ethers as solvents for the 1,3-butadiene gives equally goodresults as does use of aliphatic or aromatic hydrocarbons.Representatives of ethers are, for example, diethyl ether or methyltert-butyl ether.

It has been found to be advantageous for one part by volume of1,3-butadiene to be dissolved in at least one part by volume of theinert medium. Preferably, one part by volume of 1,3-butadiene isdissolved in from one to ten parts by volume of the inert medium.

A particularly advantageous embodiment of the process of the inventioncomprises using 1,3-butadiene in the form of industrial hydrocarbonmixtures which mixtures are unavoidably obtained in considerableamounts, for example, as refinery by-products in the production ofautomobile fuel and in the preparation of ethylene by thermal crackingof higher hydrocarbons (C₄ cracking fractions). In addition to1,3-butadiene and saturated C₄ -hydrocarbons, they also containn-but-1-ene, isobutene and cis- and trans-n-but-2-ene. Under theconditions of the butadiene hydroformylation, the olefins mentioned,with the exception of n-but-1-ene, are largely inert and together withthe saturated C₄ -hydrocarbons n- and i-butane form the inert medium inwhich the 1,3-butadiene is dissolved according to the invention.

The reaction of industrial hydrocarbon mixtures containing 1,3-butadienein the process of the invention thus allows not only the preparation ofolefinically unsaturated C₅ -aldehydes, but in the context of a novelcarborefining process leads to a raffinate comprising isobutene, cis-and trans-n-but-2-ene and the saturated C₄ -hydrocarbons and containsonly traces of 1,3-butadiene and n-but-1-ene. Isobutene can be separatedfrom this mixture by known methods, e.g. by conversion into methyltert-butyl ether. The remaining C₄ -hydrocarbons are either recirculatedto the pyrolysis process or chemically reacted in another way.

The hydroformylation of the 1,3-butadiene dissolved in the inert mediumis carried out as a heterogeneous reaction in a two-phase system, areaction which is described, for example, in DE-C 26 27 354. Thisprocess is distinguished by the presence of an organic phase whichcontains, inter alia, the olefinic compound and the reaction product andof an aqueous phase in which the catalyst is dissolved. The catalystsused are water-soluble rhodium complexes containing water-solubleorganic phosphines as ligands. The water-soluble organic phosphine isusually present in excess, based on rhodium, i.e. not only therhodium/phosphine complex but also free phosphine are present in thecatalyst system.

For the purposes of the invention, the term "water-soluble, organicphosphines" means monophosphines or polyphosphines in which alkylsand/or aryls are bonded to the trivalent phosphorus atom or atoms, withat least one of these alkyls and/or aryls being singly or multiplysulfonated or carboxylated. The preparation of such phosphines is knownand described, for example, in DE-C 26 27 354 and DD Patent 259 194.

Water-soluble organic phosphines which have been found to beparticularly useful are compounds of the formula ##STR1## wherein Ar¹,Ar², Ar³ are individually phenyl or naphthyl, X¹, X², X³ areindividually sulfonate (--SO₃ ⁻) and/or carboxylate (--COO⁻), Y¹, Y², Y³are individually selected from the group consisting of alkyl of 1 to 4carbon atoms, alkoxy, halogen, --OH, --CN, --NO₂ and (R¹ R²)N in whichR¹ and R² are individually alkyl of 1 to 4 carbon atoms, m₁, m₂, m₃ areindividually integers from 0 to 5, with the proviso that at least onem₁, m₂ or m₃ is greater than 0, n₁, n₂, n₃ are individually integersfrom 0 to 5, M is an alkali metal ion, one chemical equivalent of analkaline earth metal ion or zinc ion, an ammonium or quaternary ammoniumion of the formula N(R³ R⁴ R⁵ R⁶)⁺ in which R³, R⁴, R⁵, R⁶ areindividually hydrogen or alkyl of 1 to 4 carbon atoms.

Phosphines of Formula I which have been found to be particularly usefulare those in which Ar¹, Ar², Ar³ are each phenyl and X¹, X², X³ are eachsulfonate in the meta position relative to phosphorus, i.e. salts oftris(m-sulfonatophenyl)-phosphine (TPPTS).

The phosphine present in the catalyst system need not be uniformchemical compounds but can have different chemical compositions. Thus,they can differ, for example, in the type and bonding of the groupattached to the phosphorus atom, in the degree of sulfonation orcarboxylation or in the type of cations.

The catalyst system can be preformed before addition to the reactionmixture. However, equally good results are obtained by preparing it inthe reaction mixture under reaction conditions from the rhodium orrhodium compound and the phosphine. Apart from metallic rhodium infinely divided form, it is possible to use rhodium salts such asrhodium(III) sulfate, rhodium(III) nitrate, rhodium(II) acetate, rhodiumacetylacetonate, rhodium oxides or other carbonyl-forming compounds ofrhodium as rhodium source.

The rhodium concentration in the aqueous catalyst phase is from 100 to600 ppm by weight, preferably from 300 to 500 ppm by weight, based onthe solution. The phosphine is usually used in such an amount that atleast 20 moles, preferably from 40 to 80 moles, of P(III) are presentper mole of rhodium. The reaction of 1,3-butadiene with hydrogen andcarbon monoxide is carried out at temperatures of from 100° to 180° C.,preferably from 110° to 120° C., and pressures of from 5 to 35 MPa,preferably from 4 to 8 MPa.

The pH of the catalyst solution is from 4 to 10, preferably from 5.5 to9.

The ratio of carbon monoxide to hydrogen in the synthesis gas can bevaried within wide limits. In general, the synthesis gas used is one inwhich the volume ratio of carbon monoxide to hydrogen is 1:1 or deviatesonly little from this value.

The volume ratio of catalyst phase to organic phase can be varied in therange of 1:4 to 4:1, with a ratio of from 1:1 to 2:1 being advantageous.

The reaction can be carried out either batchwise or continuously.

The reaction product is worked up by separating the organic phase andthe aqueous catalyst phase from one another. The hydroformylationproducts are isolated by distillation from the catalyst phase.

In the following examples, there are described several preferredembodiments to illustrate the invention. However, it is to be understoodthat the invention is not intended to be limited to the specificembodiments.

EXAMPLE 1 (Comparative Example)

A 160 ml V4A stainless steel autoclave fitted with a pressure-resistantmetering vessel, a pressure sensor, a bursting disk, a thermocouple anda magnetically coupled propellant stirrer was charged with 40 g of anaqueous catalyst solution of 16.4 mg of rhodium (in the form of anaqueous rhodium(III) sulfate solution), corresponding to 410 ppm of Rhin the solution, and 3.77 g of Na₃ -TPPTS under an argon atmosphere. ThepH was adjusted to 6 by addition of Na₂ CO₃ and the autoclave wasclosed. 20 g of 1,3-butadiene from a pressure gas bottle were thencondensed into an evacuated metal capillary provided with two valves. Asinternal standard for gas-chromatographic analysis (GC analysis), aweighed amount of propane was additionally added. The metal coilcontaining the 1,3-butadiene/propane sample was screwed onto themetering vessel and the catalyst solution was added after flushing theautoclave with water gas (CO/H₂ =1/1) preformed for 90 minutes at 110°C. and 10 MPa. The desired reaction pressure for the hydroformylation,viz. 6 MPa, was then set in the autoclave. The liquefied gas in themetal coil was injected into the intensively stirred autoclave and thereaction was thereby started. After a reaction time of 5 hours, thereaction mixture was cooled, and the amount and composition of the gasphase was measured for determining the conversion. The aqueous catalystphase, after separating off the organic reaction product, was extractedthree times with 15 ml each time of diethyl ether. The solution of thereaction product in the ether was dried over Na₂ SO₄ and then wasanalyzed by gas chromatography.

    ______________________________________                                        Result:                                                                       ______________________________________                                        Conversion:        89%                                                        Composition:                                                                  n-Pentenals/n-pentanal                                                                           34%                                                        2-Propenylhepatadienal                                                                           34%                                                        2-Methylbutanal     2%                                                        Other products*)   11%                                                        Stand oil (>C.sub.10)                                                                            19%                                                        ______________________________________                                         *)This was mainly formylcyclopentene, the intramolecular aldol                condensation product of 1,6hexanedial which was formed by hydroformylatio     of npenten-4-al. Very small amounts of 2methyl-pentanedial and                ethylbutanedial are also formed as further bishydroformylation products. 

EXAMPLE 2

1,3-Butadiene was reacted in the presence of diethyl ether as solvent(volume ratio of solvent/butadiene=2) under the conditions of Example 1,but with the reaction time shortened to 4 hours.

    ______________________________________                                        Result:                                                                       ______________________________________                                        Conversion:        82%                                                        Composition:                                                                  n-Pentenals/n-pentanal                                                                           85%                                                        2-Propenylheptadienal                                                                            1.5%                                                       2-Methylbutanal    1.5%                                                       Other products     10%                                                        Stand oil (>C.sub.10)                                                                             2%                                                        ______________________________________                                         Composition of the npentenals/n-pentanal fraction: 72% of cis/trans           3pentenal, 3% of 4pentenal, 1% of 2pentenal and 23% of npentanal.        

EXAMPLES 3 TO 6

The experiments described in the Examples 3 to 12 below were carried outusing a C₄ fraction obtained by pyrolysis of naphtha and having thefollowing composition:

    ______________________________________                                        1,3-Butadiene            46%                                                  i-Butene                 23.5%                                                n-But-1-ene              11.5%                                                trans-n-But-2-ene        5%                                                   cis-n-But-2-ene          4%                                                   1,2-Butadiene, butenyne, butyne                                                                        1%                                                   n-Butane                 6%                                                   i-Butane                 3%                                                   ______________________________________                                    

In Examples 3 TO 6, the reaction temperature and the reaction time werevaried, and the starting material (20 g of C₄ fraction) and the otherreaction conditions were the same as those of Example 1.

    __________________________________________________________________________    Examples 3 to 6:                                                              Composition of the reaction products (% by weight)                            Reaction               2-                                                                              3-                                                                              Formyl-                                                                           Stand                                             Temp.                                                                              Time                                                                             n-Pentenals/                                                                        2-Propenyl-                                                                         Methyl-                                                                           cyclo-                                                                            oil                                            Ex.                                                                              °C.                                                                         h  n-Pentanal*                                                                         heptadienal                                                                         butanal                                                                           pentene                                                                           (>C10)                                                                            Remainder**                                __________________________________________________________________________    3  200  6  67(49)                                                                              6     3 1 6   5                                              4  110  4  68(62)                                                                              9     4 1 7   2                                              5  120  2  69(73)                                                                              6     4 1 6   3                                              6  120  12 57(18)                                                                              10    5 6 8   2                                              __________________________________________________________________________     *The proportion of cis/transn-penten-3-al in the C.sub.5 fraction is show     in brackets                                                                   **The remainder to 100% comprised various unidentified compounds              containing less than 10 carbon atoms in the molecule.                    

    ______________________________________                                        Examples 3 to 6:                                                              Conversion of the C.sub.4 constituents of the                                 starting material (% by weight)                                                                  cis-  trans-                                               Example 1,3-Butadiene                                                                            n-But-1-ene                                                                             n-But-2-ene                                                                            i-Butene                                ______________________________________                                        3       90         63        10    7    11                                    4       91         79         5    <1   11                                    5       91         69         1    0     9                                    6       94         93        23    5    24                                    ______________________________________                                    

EXAMPLES 7 TO 9

In Examples 7 to 9, the hydroformylation of the C₄ fraction was carriedout with variation of the P/Rh ratio in the aqueous catalyst solution.The other conditions were the same as those in Example 1, with theexception of the reaction time which was 4 hours.

    __________________________________________________________________________    Examples 7 to 9:                                                              Composition of the reaction products (% by weight)                                              2-                                                                              3-      Stand                                                P/Rh                                                                             n-Pentenals/                                                                        2-Propenyl-                                                                         Methyl-                                                                           Formyl-                                                                             oil                                               Ex.                                                                              ratio                                                                            n-Pentanal*                                                                         heptadienal                                                                         butanal                                                                           cyclopentene                                                                        (>C10)                                                                            Remainder**                                   __________________________________________________________________________    7  10 63(66)                                                                              6     4 <1                                                                              7     9                                                 8  40 68(62)                                                                              9     4  1                                                                              7     2                                                 9  60 68(59)                                                                              10    3  1                                                                              6     3                                                 __________________________________________________________________________     *The proportion of cis/transn-penten-3-al in the C.sub.5 fraction is show     in brackets                                                                   **The remainder to 100% comprised various unidentified compounds              containing less than 10 carbon atoms in the molecule.                    

    ______________________________________                                        Examples 7 to 9:                                                              Conversion of the C.sub.4 constituents of the                                 starting material (% by weight)                                                                  cis-  trans-                                               Example 1,3-Butadiene                                                                            n-But-1-ene                                                                             n-But-2-ene                                                                            i-Butene                                ______________________________________                                        7       80         34        1     0     5                                    8       92         79        5     0    11                                    9       91         68        12    9    19                                    ______________________________________                                    

EXAMPLES 10 TO 12

Examples 10 to 12 show the influence of the pH of the aqueous catalystphase on the hydroformylation of the C₄ fraction. These examples wereagain carried out under the reaction conditions given for Example 1, butwith the reaction time being shortened to 4 hours.

    __________________________________________________________________________    Example 10 to 12:                                                             Composition of the reaction products (% by weight)                                              2-                                                                              3-      Stand                                                   n-Pentenals/                                                                        2-Propenyl-                                                                         Methyl-                                                                           Formyl-                                                                             oil                                               Ex.                                                                              pH n-Pentanal*                                                                         heptadienal                                                                         butanal                                                                           cyclopentene                                                                        (>C10)                                                                            Remainder**                                   __________________________________________________________________________    10 5  81(65)                                                                              2     3 1 6     <1                                                11 7  74(64)                                                                              5     3 1 7     3                                                 12 9  68(62)                                                                              9     4 1 7     2                                                 __________________________________________________________________________     *The proportion of cis/transpenten-3-al in the C.sub.5 fraction is shown      in brackets                                                                   **The remainder to 100% comprised various unidentified compounds              containing less than 10 carbon atoms in the molecule.                    

Various modifications of the process of the invention may be madewithout departing from the spirit or scope thereof and it is to beunderstood that the invention is intended to be limited only as definedin the appended claims.

What we claim is:
 1. In the process for heterogeneous hydroformylationof 1,3-butadiene with carbon monoxide and hydrogen to form pentenals,the improvement comprising effecting the hydroformylation with1,3-butadiene dissolved in an inert reaction medium at 100° to 180° C.at 5 to 35 MPa in the presence of an aqueous solution of arhodium/phosphine complex and optionally a further phosphine ascatalyst.
 2. The process of claim 1 wherein the inert medium is a memberof the group consisting of aliphatic and cycloaliphatic hydrocarbons of3 to 10 carbon atoms and aromatic hydrocarbons of 6 to 9 carbon atoms.3. The process of claim 1 wherein the inert medium is an ether.
 4. Theprocess of claim 1 wherein the solution of 1,3-butadiene in an inertmedium is a mixture of C₄ -hydrocarbons obtained in the refining ofcrude oil.
 5. The process of claim 1 wherein 1 part by volume of the1,3-butadiene is dissolved in at least 1 part by volume of the inertmedium.
 6. The process of claim 5 wherein 1 part by volume of the1,3-butadiene is dissolved in from 1 to 10 parts by volume of the inertmedium.
 7. The process of claim 1 wherein the rhodium/phosphine complexcontains a monophosphine of the formula ##STR2## where Ar¹, Ar², Ar³ areindividually phenyl or naphthyl, X¹, X², X³ are individually a sulfonate(--SO₃ ⁻) and/or carboxylate (--COO⁻) Y¹, Y², Y³ are individuallyselected from the group consisting of alkyl of 1 to 4 carbon atoms,alkoxy, halogen, --OH, --CN, --NO₂ and (R¹ R²)N wherein R¹ and R² areindividually alkyl of 1 to 4 carbon atoms, m₁, m₂, and m₃ areindividually integers from 0 to 5, with the proviso that at least onem₁, m₂, or m₃ is greater than 0 and n₁, n₂, n₃ are individually integersfrom 0 to 5, M is selected from the group consisting of an alkali metalion, one chemical equivalent of an alkaline earth metal ion or zinc ion,an ammonium and quaternary ammonium ion of the formula N(R³ R⁴ R⁵ R⁶)⁺in which R³, R⁴, R⁵, R⁶ are individually hydrogen or alkyl of 1 to 4carbon atoms.
 8. The process of claim 7 wherein the rhodium/phosphinecomplex contains a tris(m-sulfonatophenyl)phosphine salt.
 9. The processof claim 1 wherein the reaction is carried out at 110° to 120° C. and at4 to 8 MPa.
 10. The process of claim 1 wherein the rhodium concentrationin the aqueous catalyst solution is from 100 to 600 ppm by weight, basedon the solution.
 11. The process of claim 1 wherein at least 20 moles ofP(III) in the form of a phosphine are present per mole of rhodium. 12.The process of claim 1 wherein the pH of the aqueous catalyst solutionis from 4 to
 10. 13. The process of claim 12 wherein the pH of theaqueous catalyst solution is from 5.5 to
 9. 14. The process of claim 1wherein the volume ratio of aqueous to organic phase is from 1:4 to 4:1.15. The process of claim 14 wherein the volume ratio of aqueous toorganic phase is from 1:1 to 1:2.
 16. The process of claim 11 wherein 40to 80 moles of P(III) in the form of phosphine are present per mole ofrhodium.
 17. The process of claim 10 wherein the rhodium concentrationin the aqueous catalyst solution is 300 to 500 ppm by weight, based onthe solution.